Ultrafast photo-induced nuclear relaxation of a conformationally disordered conjugated polymer probed with transient absorption and femtosecond stimulated Raman spectroscopies

Abstract

A combination of transient absorption (TAS) and femtosecond stimulated Raman (FSRS) spectroscopies were used to interrogate the photo-induced nuclear relaxation dynamics of poly(3-cyclohexyl,4-methylthiophene) (PCMT). The large difference in inter-ring dihedral angles of ground and excited-state PCMT make it an ideal candidate for studying large-amplitude vibrational relaxation associated with exciton trapping. Spectral shifting in the S1 TA spectra on sub-ps timescales (110 ± 20 and 800 ± 100 fs) is similar to spectroscopic signatures of excited-state relaxation observed with related photoexcited conjugated polymers and which have been attributed to exciton localization and a combination of resonant energy transfer and torsional relaxation, respectively. Measurements made with both techniques reveal fast PCMT S1 decay and triplet formation (τS1 = 25-32 ps), which is similar to the excited-state dynamics of short oligothiophenes and highly twisted polyconjugated molecules. On ultrafast timescales FSRS of S1 PCMT offers a new perspective on the nuclear dynamics that underlie localization of excitons in photoexcited conjugated polymers: Spectral dynamics in the C=C stretching region (1400-1600 cm(-1)) include a red-shift of the in-phase C=C stretching frequency, as well as a change in the relative intensity of in-phase and out-of-phase stretch intensities on a timescale of ∼100 fs. Both changes indicate an ultrafast vibrational distortion that increases the conjugation length in the region of the localized excitation and are consistent with exciton self-localization or trapping. Wavelength-dependent excited-state FSRS measurements further demonstrate that the C=C stretching frequency provides a useful spectroscopic handle for interrogating the degree of delocalization in excited conjugated polymers given the selectivity achieved via resonance enhancement.

Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy

“…Changes in the electron density influence the electron delocalization and hence cause a reduction in the force constant of the vibrational frequencies at sub-picosecond time scales. [45][46][47] The observed rate of the slower frequency upshift also differs for the CC triple and aromatic CC double bonds, as shown in Table III. We have assigned the 8.6 ps and 12.1 ps components as vibrational relaxation (cooling), which could be coupled with torsional degrees of freedom by transferring energy to torsional degrees of freedom and may contribute to the rate of planarization.…”

Femtosecond Stimulated Raman Spectroscopy

“…While these fixed excitation wavelengths are suitable for some applications, in general it is desirable to have fully tunable actinic pulses. This can be achieved by either using an (IR‐) OPA with subsequent harmonic generation for UV‐FSRS or directly in the visible by using a dedicated fs‐NOPA. A good overview of NOPA designs has been given by Cerullo and De Silvestri, which now routinely yield tuning ranges from 460 to 750 nm and pulse widths on the order of 20 fs after compression with prisms or chirped mirrors.…”

Stimulated Raman Scattering: From Bulk to Nano

“…Examples of research in which these SRS techniques are employed include studies of polymeric photovoltaic systems, 124–126 charge transfer complexes, 127–129 DNA base pairs, 130 and other photoactive proteins. 131–133 …”